In February 1951, in Korea, while the armistice negotiation still faces difficulties, the battle continues in the Eastern front line, on the Aero.K. A company commander of the South Korean army dies in battle, and the bullet found in his body belongs to the South Korean army. Lieutenant of the Defense Security Command KANG Eun-pyo(SHIN Ha-kyun) is sent to the Eastern front line to investigate if this is related to any collusion with the enemy.